Fungal infections are a major cause of morbidity and mortality in the immunocompromised patient. Until recently, available antifungals were limited by systemic toxicity (polyenes and imidazoles), lack of fungicidal activity (triazoles and flucytosine), or narrow spectrum (allylamines). Echinocandins (ECs) such as the recently FDA-approved caspofungin represent a new antifungal class that appears to address these limitations. ECs inhibit the fungal specific enzyme beta-1,3-glucan synthase responsible for synthesizing a major cell wall polysaccharide. While common fungal pathogens such as Candida albicans and Aspergillus fumigatus are susceptible to ECs, others such as Cryptococcus neoformans are intrinsically resistant for reasons that are unclear. Also, there is an undefined potential for the selection of EC-resistant or tolerant mutants of normally susceptible fungi. Using Saccharomyces cerevisiae as genetic model, initial experiments identified unique sets of "ECH" genes which, when overexpressed or deleted, conferred EC resistance or hypersensitivity. These genes encode Golgi or plasma membrane-associated proteins known to play roles in cell wall synthesis, as well as protein kinases, transcription factors, and other proteins with no previous connection to this process. But do these S. cerevisiae results have relevance to EC activity in clinically important fungi? This R03 proposal will examine this through the following Specific Aims: (1) Complete the identification of S. cerevisiae ECH genes by replica plate screening for hypersensitive deletion mutants, and test all mutants for altered antifungal sensitivities and other relevant phenotypes. (2) Construct C. albicans strains with deletions in representative ECH homologs and test for altered susceptibilities to ECs and other antifungals, and for other relevant phenotypes such as altered yeast-hyphal morphogenesis. (3) Select in vitro for C. albicans spontaneous EC-resistant or tolerant mutants (initial studies revealed a high frequency of the latter) and examine for altered gene expression by RNA hybridization and altered gene products by DNA sequencing. Building on this foundation, future studies are likely to include: (1) testing the in vivo susceptibility and virulence of these EC-resistant or tolerant mutants, (2) identifying drugs which act synergistically or antagonistically with ECs and determining the basis for these interactions, (3) characterizing mechanisms for acquired and intrinsic EC resistance in clinical isolates.